Currently, passive optical network (PON) systems continue to deliver content to homes and offices across the world. Increasing bandwidth and data content demands have caused newer signaling protocols and corresponding data speeds to emerge. Interference signaling and signal degradation remains a known concern in PONS and next generation gigabit (XG)PON-type systems. In one example, Raman cross-talk is believed to occur from lower wavelengths into higher wavelengths. For instance, a GPON operating at 1490 nm may cause Raman cross-talk into a 1550 nm video overlay service.
One known implementation may include the use of GPON payload scrambling and using a lower GPON transmit power (approximately +5 dBm) to achieve acceptable performance at the optical network termination units (ONTs). Raman cross-talk may also occur at higher wavelengths that traverse into lower wavelengths, such as from 1577 nm into 1550 nm. Though the wavelength spacing is close, which in turn, results in a lower Raman coupling coefficient, the XGPON-1 power spectral density may be reduced since the data rate is 10 Gbps, which implies less power on a per-Hz basis. As a result, the +12.5 dBm optical transmitter power level still results in video service degradation when following transmission over the ODN (i.e., 10-20 km of fiber and splitter loss). The optical input level to an ONT is on the order of −12 dBm. Under these conditions the Raman cross-talk is a significant factor in the recovered carrier-to-noise ratio (CNR), signal-to-noise ratio (SNR), and modulation error ratio (MER) for the first few recovered video channels (55 MHz-120 MHz).
The above-noted performance criteria may be reduced to levels incompatible with network deployment guidelines. In the case of digital video (256 QAM) the bit error rate (BER) may be reduced to unacceptable levels. Unacceptable performance levels impact video customer service by placing impairments or complete loss of recovered video service on some channels. Some known ways to mitigate the Raman cross-talk impact upon the video data include significantly reducing the XGPON-1 overall transmit power level, and using pre-emphasis on the lower video channel modulation applied to the 1550 nm head-end video transmitter.
Reducing the power transmission results in the inability of the XGPON-1 service to have the desired link budget or service distance. Modifying the transmitters requires modifications to existing deployed PON systems and re-configuring thousands of 1550 nm optical video transmitters. As a result, the existing options for reducing Raman cross-talk include unfeasible service restrictions and/or expense and complex upgrades which are commercially unacceptable and may also lead to backwards compatibility issues with existing deployments.